1. Field of the Invention
The present invention relates to an electronic musical instrument, and particularly to an electronic musical instrument that produces a desired tone signal by employing multiple oscillators.
2. Description of the Related Art
Nowadays, various electronic musical instruments, such as electronic keyboards, electronic pianos, and synthesizers, have been developed and are in practical use. Such electronic musical instruments employ multiple oscillators, for example, 16 or 32 oscillators, to produce basic portions of musical tones.
Musical tones produced by the oscillators, however, are monotonous, and the emotional, rich sounds obtained when playing natural musical instruments cannot be expected. Most electronic musical instruments, therefore, have sub-oscillators that produce sound effects, such as vibrato, tremolo, and growl, and an envelope generator that adjusts volume and alters frequency bands.
In such an electronic musical instrument, musical tones that are produced by main oscillators are modulated by employing low-frequency musical tones that are produced by sub-oscillators to obtain a variety of sound effects, such as vibrato. As tone wave data are written to the oscillators, modulation wave data are written to the sub-oscillators, so that a desired musical tone is generated in response to the input from a keyboard.
This system however requires 16 sub-oscillators if it has 16 oscillators, and its circuit structure is thus complicated and expensive to fabricate. Although one method has been tested that reduces the sub-oscillator count in consonance with the maximum simultaneously produced timbre count, a satisfactory structure has not yet been designed.
These sub-oscillators, as described above, add modulation to produce sound effects, such as vibrato. Although the types of modulation frequencies employed do not have to have the same counts as oscillators, multiple similarly structured sub-oscillators are provided. Regarding the circuit structure and its cost, the wasteful practice of providing the same number of sub-oscillators as the number of oscillators or as the number of simultaneously produced timbres has been employed.
Most of the envelope generators employed for such electronic musical instruments provide target values and the speeds required to reach these values. To effect volume changes or to alter frequency bands, usually target values are changed.
On the other hand, as the variety of available electronic musical instruments has increased, so too has the demand for electronic musical instruments with improved ease of manipulation. Especially required are real time techniques for modulation control, such as those that can be employed for volume control and frequency band control.
Since a conventional system, however, cannot control such modulations in real time, it has to employ many additional circuits, such as a multiplier, a switching circuit, a table conversion circuit, and an interpolation circuit. As its circuit size is thus enlarged, its cost is consequently increased.